The Genera Rhodothermus, Thermonema, Hymenobacter and Salinibacter
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The Genera Rhodothermus, Thermonema, Hymenobacter and Salinibacter AHARON OREN
Introduction The genera Rhodothermus, Thermonema, Hymenobacter, and Salinibacter all consist of Gram-negative, red- or yellow-pigmented obligatory aerobic chemoheterotrophic bacteria. The first two genera are thermophilic, with temperature optima around 60–65∞C; Hymenobacter and related strains described in the past as “Taxeobacter” species are oligotrophic soil bacteria, some of them isolated from cold environments in the Antarctic. Finally, Salinibacter is a genus of extremely halophilic red bacteria that are among the most salt-tolerant and salt-requiring strains within the bacterial domain. Phylogenetically they all form deep branches affiliated with the Cytophaga-Flavobacterium-Bacteroides branch of the Bacteria (Fig. 1 shows Thermonema and Rhodothermus, Figs. 2 and 3 show Hymenobacter and Salinibacter). While relatively little is known about the properties of Thermonema and Hymenobacter, extensive research has been devoted to Rhodothermus in view of its production of thermotolerant cellulases, xylanases, and other biotechnologically potentially valuable thermophilic enzymes. The extremely halophilic Salinibacter was only recently discovered, but it has become evident that Salinibacter makes a significant contribution to the biota of NaCl-saturated saltern crystallizer ponds. The species is of considerable interest in view of its mode of osmotic adaptation, which differs from that of the other aerobic halophiles within the domain Bacteria. Unexpectedly, it shares many properties with the extremely halophilic representatives of the archaeal order Halobacteriales.
The Genus Rhodothermus Rhodothermus marinus, the type species of the genus Rhodothermus and presently the only rec-
ognized species of the genus, was first isolated from shallow-water submarine hot springs in Iceland (Alfredsson et al., 1988; Kristjánsson and Alfredsson, 1992). It is a thermophile that grows optimally at 65∞C and tolerates up to 77∞C for growth. Similar isolates have since been obtained from a variety of thermal environments on the Azores (Nunes et al., 1992a) and in Japan (Sako et al., 1996). Since it was discovered that Rhodothermus strains possess thermophilic cellulase and xylanase activities, as well as other thermophilic enzymatic activities of potential biotechnological interest, much study has been devoted to the genus.
Taxonomy and Phylogeny The type strain of Rhodothermus marinus (Alfredsson et al., 1988) is DSM 4252, ATCC 43812 (16S rDNA accession number X80994). The G+C content of its DNA is 64.4–64.7 mol%. A second species, R. obamensis, was proposed by Sako et al. (1996) to accommodate a new isolate from a shallow marine hydrothermal environment in Tachibana Bay, Nagasaki Prefecture, Japan. The proposed type strain (OKD7 = JCM 9785) was claimed to differ sufficiently from R. marinus to warrant its description as a new species. The G+C content of its DNA was higher (66.6 m
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